Obviously each difficult mother/mother-in-law will have their own background, so their own reasons for being difficult.
Equally, all of the next generation will have their own reasons for reacting differently to them. For example, my mother was very controlling. I have pretty much escaped from her, so find the controlling ways of my MIL to be a huge imposition.
But, my mother is/was controlling because she has (recently diagnosed) Aspergers, so needs a firm set of rules to live by. MIL is from a dysfunctional family with a strict hierarchy. When I joined the family, it was very important to her to ensure that I was not higher up the pecking order than she was.
And to be fair to them, neither could help the reason they are controlling.
But I think it does go deeper than "every case is different", and is deeply rooted in ageism and sexism.
Men in our society, and therefore in most families, are more important than women. However, women of a childbearing age have an importance, as mothers. Often women discover a respect when they become mothers that they hadn't been given before. As they get older, this may diminish, particularly if they have been a SAHM, so have no perceived economic value in society. That is, they are no longer seen as important as a mother, or for any other reason.
So, they cling on to the title of "mother", as this is what has given them an importance that they don't wish to lose. For a MIL, their child's partner may be seen as usurping that position, so they may use very underhand methods to ensure that that doesn't happen.
In the case of a mother, it tends to be more a reluctance to allow their child to grow up, or to be perceived as a grown up. So, mothers may often be very critical of their grown children's choices, and wish to have a lot of input into those choices, to ensure their continued importance in their child's life.
Of course, in real life, where parents allow their children to grow up, and walk away, and make their own life, they are more often the ones who do manage to retain a happy relationship with their grown children. (Not always, though, as of course, those grown children may well be "difficult" in their own right.)
(This is my theory, and is in no way original, so if these ideas are similar to ones you may have written or seen on MN before, it's because I am in agreement with them.)